This invention relates to a new and improved light pen system for use with graphic displays. More particularly, it relates to a light pen having a new and improved design which permits continuous control of light pen output based on user applied pressure.
In the prior art, light pens have been used as a pointer device in association with computer controlled cathode-ray tube video displays. The pen is most often an object about the size and shape of a fountain pen and inlcudes a means for sensing light and a means for converting this light into an electrical pulse. The pen is pointed by hand to some portion of the picture on the screen, and when the electron beam which is tracing the image causes that portion of the sceen to light up, the pen senses the light and generates an electrical pulse which serves as a signal to the computer. Typically, the computer circuit responds to this signal by reading an address counter having data which tracks the position of the cathode ray at that particular instant. The address in the counter, thus, corresponds to the location on the screen to which the light pen points.
A light pen typically includes an elongated channel for directing light rays emanating from the video to a photodetector within the light pen. One example of a light pen can be found in U.S. Pat. No. 3,569,617 issued to Allen on Mar. 9, 1971. The light pen disclosed by Allen includes a collimation tube having an internal channel with non-reflective walls. Only light rays that are axially aligned with the channel are transmitted from the video screen to the photodetector. Non-axial light rays emitted from the screen are either not received through the channel opening or absorbed by the non-reflective walls of the channel. Various other light pens have been developed which include a lens for focusing the axially received light. Examples of such pens can be found in U.S. Pat. No. 3,271,515 issued Sept. 6, 1966 to Harper, U.S. Pat. No. 3,917,955 issued Nov. 4, 1975 to Inuiya and U.S. Pat. No. 4,454,417 issued June 12, 1984 to May.
These pens rely heavily on the ability to detect only the light coming from a small area. In addition to the issue of accepting narrow beams of light from the video screen, prior art light pens also address the issue of light pen activation. A light pen is normally kept electrically disconnected, or off, so that stray light impinging on the pen does not send an unwanted signal to the computer. Only when the light pen needs to be active does the user normally turn the pen "on". Although some light pens employ separate mechanical means to actuate the pen, improved means are disclosed in the Allen, Inuiya and May patents, in which the collimation tube is slidably mounted within an outer housing. When the user presses the collimation tube against the video screen, a miniature switch in the rear of the housing is actuated, thereby signaling the computer that the light pen is active.
None of the prior art light pens permit light from large areas to be accepted and none have means for controlling the electrical output of the pen in response to the pressure applied thereon, save for the on-off switching done by the aforementioned light pens. It is an object of this invention, therefore, to provide simple and effective means for translating the pressure applied on a light pen into a continuously variable control of the light pen output.